Pre-distortion is used in transmission systems to compensate for the linear and nonlinear effects of the transmission channel upon the signals to be transmitted. An adaptation engine may generate an error correction signal for a pre-distortion circuit. The error correction signal causes the pre-distortion circuit to modify the input signal in a way that counteracts the transmission channel response. As a result, the system output signal should be equivalent to the input signal with some gain value applied without other modification. The adaptation engine must know the transmission channel response in order to generate the correct error correction signal. The transmission channel response can be measured using external monitoring equipment that inputs a known signal and analyzes the output after passing through the transmission channel. The use of such external measuring equipment is not practical when the system is in use outside a production or test environment.
An adaptation engine internal to the system can also be used to measure a transmission channel response. The adaptation engine receives both the system input signal and the system output signal and then compares the input and output signals to determine the transmission channel response. As a result, the adaptation engine can determine the transmission channel response for current operating conditions. However, in such systems, the system output signal is provided to the adaptation engine via a feedback channel. Because the output signal must be down-converted, mixed, filtered or otherwise modified in the feedback channel before being applied to the adaptation engine, the feedback channel introduces its own response to the output signal in addition to the transmission channel response. Accordingly, the adaptation engine generates an error correction signal designed to counteract both the transmission channel response and a feedback channel response. Only the transmission channel portion of the pre-distortion will have been neutralized when the signal reaches the system output. As a result, the output signal will still include the inverse of the feedback channel, which was unintentionally included in the error correction signal from the adaptation engine. The feedback channel response must be identified by the system and eliminated from the pre-distortion correction.